The present invention relates to a process for coating metallic substrates with a powder coat or a liquid coat including the pre-treatment of the substrate surface.
Using powder coats to provide decorative or functional coating of surfaces has been widely used in the coating of metals due to the high economic efficiency of the process as well as its wide acceptance from the point of view of environmental protection. Numerous powder coat compositions have been developed for different applications. The known processes for coating with powder coats require that the metal surface to be coated must be subjected to a costly, generally aqueous pre-treatment process. Without such pre-treatment, the adherence of powder coat layers and hence the corrosion protective effect is insufficient. Such costly pre-treatments are also necessary for the application of aqueous or solvent-containing liquid coats on metal surfaces.
After their application, powder and liquid coats can be cured, or dried and cured, by means of radiation having wavelengths in the near infrared (NIR) region. In the field of powder coating, NIR-technology enables melting and curing of powder coats to be carried out in a single process step, see for example K. Bar, JOT 2/98, pp. 26 to 29 and WO 99/41323. A uniform heating of the whole coating layer is achieved. Such NIR-curing processes also require costly pre-treatment methods for the substrate surface prior to coating.
The pre-treatment of steel or aluminium consists generally of at least several processing steps. First of all, the removal of fats, oils or other impurities is carried out, for example by an alkaline washing using a dip or spray technique. A subsequent interim rinse is generally followed by a wet-chemical deposit of inorganic corrosion protection layers on the surface (for example, phosphating in the case of steel, chromatising in the case of aluminium). A rinsing operation with fresh water or demineralised water is again required in order to completely remove adhered reaction chemicals. The damp metal surface must be dried completely before the subsequent application of the powder or liquid coat layer in order to avoid coating faults. Typical pre-treatment methods for the powder coating of metal surfaces are described for example in T. Molz, Tagungsband der DFO-Pulverlacktagung, 23.-24.9.1996, pages 201 to 207.
This pre-treatment of metal substrates or metal parts prior to coating with powder or liquid coats requires large dip coating tanks or spraying cabins involving considerable investment and operating costs. The chemicals used in the different reaction zones must be kept strictly separate from each other, and spent pre-treatment solutions must be disposed of at high cost. If faults occur in the pre-treatment, faults in the subsequent coating are unavoidable.